1. Field of the Invention
This invention relates to optical systems for television cameras and, more particularly, to the tracking adjustment of a television camera using a solid-state image sensor.
2. Description of the Related Art
In a television camera, light from the objective lens enters the color separation prism to produce three beams of red, green and blue colors with which images are formed on the respective image sensors, and color signals are obtained.
A tracking discrepancy arises from the difference in length between two of the optical paths of the light beams passing through the color separation prism from the objective lens depending on manufacturing errors. Also, because the objective lenses have generally different residual longitudinal chromatic aberrations, the tracking is further degraded if as the lens type is interchanged with another one.
In the past, a television camera employed the tube type image sensor in combination with a mechanism for tracking adjustment included in each of the mechanisms for keeping hold of the image pickup tubes. And, it was usual that its range of adjustment was wide enough to absorb the possible maximum error of tracking.
In recent years, however, as the image sensing device, increasing numbers of the solid-state image sensors have been used in place of the image pickup tube. Since that solid-state image sensor, as compared with the image pickup tube, is very small in size and very light in weight, no such large scale holding mechanism (as in the case of the image pickup tube) is required to be adopt. It is, therefore, easy to simplify the mounting mechanism.
But, because, as has been described before, there is an error in the length of the optical path along which either one of the split light beams produced by the color separation prism travels, a difference results between any two of the lengths of the optical paths, by which a tracking discrepancy arises. Particularly with this origin of discrepancy, if at least two of the solid-state image sensors are fixedly mounted directly on the respective exit faces of the prism, it is difficult to bring all the tracking into coincidence.
For this reason, in the present state of the art of television cameras of the type using the solid-state image sensors, a simple holding mechanism is used to provide a gap between the solid-state image sensor and the exit face of the prim for the purpose of adjusting the tracking. After the tracking adjustment has been performed, the gap is then filled with an adhesive agent, a spacer, or molten metal, thus fixedly securing the solid-state image sensor to the exit face of the prism. In such a way, the tracking error caused by the prism system is absorbed.
In this case, however, the arrangement is such that the image sensor is fixedly mounted at the predetermined start position by using a tool lens, or the like. All that can be done is, therefore, only to absorb the error of the prism system. The before-described problem that the tracking discrepancy due to the differences of the longitudinal chromatic aberration with different lens types cannot be absorbed has been left unsolved.
Also, as the photographic lens, zoom lenses are generally used. By zooming and focusing and by varying of the aperture value, the longitudinal chromatic aberration is also varied. Because of this, in each channel, the image surface is taken out of coincidence with the image sensing plane. This appears to be a tracking error, thus deteriorating the image quality. Particularly with the solid-state image sensor which has lately been put to practical use at a rapidly increasing rate, when applied to the color television camera, it is common that such a solid-state image sensor in the form of a CCD is fixedly mounted on the color separation prism. Therefore, the static correction of longitudinal chromatic aberration which was possible to some extent in the conventional image pickup tube type camera becomes impossible. Hence, it becomes more difficult to stabilize the longitudinal chromatic aberration of the zoom lens.
To achieve this stability of longitudinal chromatic aberration, it may be considered that, for example, a glass material of large extraordinary partial dispersion, though expensive, is employed, or the number of lens elements is increased. But, either the price or the size will increase greatly.
By the way, a method of correcting the tracking by moving the image pickup tube itself to vary its relative position to the color separation prism is proposed in Japanese Patent Publication No. Sho 56-1832. But, in the case of moving the image pickup tube itself, the ratio of the amount of movement of the image pickup tube to the amount of tracking correction becomes 1:1, in other words, the responsiveness is very high. Therefore, highly accurate of correction is difficult to perform. Also, the image pickup tube must be moved in accurate axial alignment with the light beam emerging from the color separation prism. Otherwise, a position discrepancy will arise between two of the channel images, or a so-called registration error is produced. Hence, a deterioration of the image quality results.